1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., including part of, one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements which serve to generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers and scanners. In steppers, each target portion is irradiated by exposing an entire pattern onto the target portion during one pass. In scanners, each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
A lithographic apparatus is known that uses patterning devices including an array of individually controllable elements (e.g., a grating light valve). In particular, a lithographic apparatus is known in which each of the individually controllable elements is a diffractive optical MEMS device. Each diffractive optical MEMS device can include of a plurality of reflective ribbons. Alternate ribbons can be deformed, relative to the remaining ribbons, such that the undeformed ribbons form a grating (e.g., a diffraction gratin). Accordingly, in the undeformed state the diffractive optical MEMS device functions as a plane reflector, reflecting incident light. In the deformed state, the diffractive optical MEMS device functions as a grating, and the incident light is diffracted.
Using an appropriate spatial filter, the undiffracted light (i.e., the reflected light from diffractive optical MEMS devices functioning as planar reflectors) can be filtered out of the beam of radiation returned from the array, leaving only the diffracted light to reach the substrate. In this manner, the beam is patterned according to the addressing pattern of the array of diffractive optical MEMS devices. Typically, the array is matrix-addressable, using suitable electronic means.
The use of diffractive optical MEMS devices is, however, limited because each device is only capable of controlling the intensity of radiation directed onto a portion of the substrate, and cannot adjust the phase of the radiation relative to the radiation from adjacent devices.